This invention relates generally to controlling of the direction of drilling a borehole in the earth, for causing that borehole to traverse a desired path within the earth.
Early apparatus and methods used for this purpose employed a device called a whipstock that was lowered into a borehole and oriented to the direction of desired borehole divergence from its initial path. This apparatus had a tapered portion that would force the drill bit to diverge in the oriented direction. Later apparatus and methods were developed that used a down-hole motor, driven by drilling-mud flow or other means. Such motors are typically mounted to the lower end of a bent subassembly such that the longitudinal axis of the motor, and the drilling bit at its lower end, are at a slight angle to the direction of the drill string above the bent subassembly. When it is desired to drill in a generally straight path, the motor may be not activated, if desired, and drill string is continuously rotated. When it is desired to cause the path of the borehole to diverge in a given direction, continuous rotation of the drill string is stopped. Then the drill string, bent subassembly, motor and bit are rotated to position the direction of bend in the bent subassembly in the desired direction of divergence, the upper part of the drill string is held in this position and the down-hole motor is started. This causes the borehole to diverge in the desired and selected direction. Down-hole motors are expensive and have a relatively short life while drilling.
As an alternative to the use of a bent subassembly and a down-hole motor, various other apparatus and methods have been developed for steerable rotary drilling. Most, if not all of these, provide some means of providing a sideways-direction force relative to the lower end of the drill string to cause the path of the drill string to diverge from a straight path.
Three early U.S. Pat. Nos. 4,394,881, 4,635,736 and 5,038,872, disclosed two spaced-apart centralizers that were mounted to a collar by a number of bladders or other flexible elements that were fluid-filled. Fluid passages connected upper bladders to lower bladders such that if an upper was compressed on the low side of the hole, a lower one would receive pressure on the high side of the hole to force the bit down. There were no sensor elements and no gain functions in the system.
Two other rotary steering developments are disclosed in prior patents, referred to as a modulated bias unit, GB 2,259,316 and U.S. Pat. No. 5,520,255, and a control unit, GB 2,257,182, U.S. Pat. Nos. 5,265,682 and 5,695,015. This apparatus is generally described in a Schlumberger brochure, “PowerDrive, The New Direction in Rotary Drilling”.
The modulated bias unit as generally described in the brochure, is firmly attached to the drill string and bit and has piston-like members that can be pushed out to provide side force. The control unit provides control of valving for these pistons that results in cycling the actuators in the modulated bias unit to keep the force acting in a desired spacial direction, as the drill string and bit rotate. The valving for the bias units is controlled by a shaft at the output of the control unit. The shaft is stabilized in space about the rotation axis, but is not however stabilized with respect to level. The attitude of stabilization provides the direction in which the bias unit will push. The control unit basically provides a mechanical control of the bias unit. For example, the Summary in U.S. Pat. No. 5,265,682 states, “The invention also provides a steerable rotary drilling system comprising a roll stabilized instrument assembly having an output control shaft the rotational orientation of which represents a desired direction of steering . . . ”. That patent does not disclose or include a “strapped-down” configuration of sensors. The Background of the Invention states, “With the drill collar rotating, the principle choice is between having the instrument package, including the sensors, fixed to the drill collar and rotating with it, or having the instrument package remain essentially stationary as the drill collar rotates around it (a so-called “roll-stabilized” system).
In U.S. Pat. No. 5,265,682, the use of roll sensors is discussed, as follows: “As previously mentioned, the roll sensors 27 carried by the carrier 12 may comprise a triad of mutually orthogonal linear accelerometers or magnetometers”, and, “In order to stabilize the servo loop there may also be mounted on the carrier 12 an angular accelerometer. The signal from such an accelerometer already has inherent phase advance and can be integrated to give an angular velocity signal which can be mixed with the signals from the roll sensors to provide an output which accurately defines the orientation of the carrier.”
U.S. Pat. No. 5,695,015 has a similar statement about “stabilized” vs. “strapped-down”. In all of these control unit patents, the stabilization torque is obtained by vanes in the mud flow and brakes, either electrical or mechanical. Power generation is disclosed as being from the same vanes.
U.S. Pat. No. 5,803,185, entitled “Steerable Rotary Drilling Systems and Method of Operating Such Systems”, appears to combine one of the earlier bias and control units with additional hardware such that the valving in the control unit can also be used to transmit data to the surface through pressure pulses.
U.S. Pat. No. 5,842,149, entitled “Closed Loop Drilling System”, addresses steerable rotary drilling and other techniques. It shows and mentions “Directional Devices to Correct Drilling Direction”. FIG. 3 shows apparatus adjacent to the bit that can push on the sides. Such apparatus does not appear to be described as stabilized in space. The shaft for the drill bit drive appears centralized, while control elements are described as being in a non-rotating part. For example, the patent states “An inclination device 266, such as one or more magnetometers and gyroscopes, are preferably disposed on the non-rotating sleeve 262 for determining the inclination of the sleeve 262”.
U.S. Pat. No. 5,979,570 discloses an apparatus for selectively controlling, from the surface of the earth, a drilling direction of an inclined wellbore. The apparatus comprises a hollow rotatable mandrel having a concentric longitudinal bore, a single inner eccentric sleeve rotatably coupled about the mandrel and having an eccentric longitudinal bore, an outer housing rotatably coupled around the single inner eccentric sleeve and having an eccentric longitudinal bore with a weighted side adapted to seek the low side of the wellbore, a plurality of stabilizer shoes and a drive means to selectively drive the single inner eccentric sleeve with respect to the outer housing. Since the offset required to provide the desired divergence from the initial wellbore direction is created by the weighted off-center element, this apparatus is only of use in an inclined borehole and is not useful in a vertical, or near-vertical wellbore. Also, the drive means must be activated at the surface of the earth before entry of the drill string into the borehole.
U.S. Pat. Nos. 5,307,885, 5,353,884 and 5,875,859 disclose the use of one or more eccentric cylindrical members to provide for lateral displacement of a section of the drill pipe. Universal joints are used so that the direction of the bit with respect to the drill string axis of the bit can be changed by the eccentric members. The axial load on the drill bit is transferred around the segment having the universal joints through a fixed outer housing. International Application WO 01/04453 A1 discloses an approach very similar to those three patents, but the drill-pipe segment containing the universal joints is replaced by a flexible pipe section that can be directly bent by the eccentric cylindrical member. In these four patents, as well as with the previously-cited approaches using eccentric cylinders, the degree of lateral offset is controlled by differential rotation of the eccentric cylinders about the borehole axis.
All of the above prior disclosures lack the unusual advantages in construction, operation and results of the present invention.